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Sputum Culture

last authored: March 2010, David LaPierre

 

 

Introduction

Pneumonia, or infection of lung tissue, can be caused by many different bacteria, viruses, fungi, and parasites.

 

Common, readily recoverable bacteria in the community include Streptococcus pneumoniae, Staphylococcus aureus, Chlamydophilia pneumoniae, Mycoplasma pneumoniae, Hemophilus influenzae.

 

Common bacteria in the hospital, or for those on recent antibiotics, include E coli, Pseudomonas, S. aureus, and Klebsiella.

 

Normal flora, or transient colonization, includes a number of different bacteria, such as Streptococci, Neisseria, Staphylococcus aureus, anaerobes, and Haemophilus. It is vitally important to ensure samples are not contaminated with oral flora.

 

  • sputum collection
  • lab processing

Sputum collection

A sputum sample should be collected on patients suspected of pneumonia. The best sample is brought up by the first cough of the morning, and should be from deep within the lungs. Patients should rinse their mouth with water and attempt to produce sputum that has not touched their mouth.

 

Contamination by saliva and oral bacteria can lead to misleading diagnoses and should be avoided at all costs.

 

Patients on ventilators can often develop pneumonia. Respiratory secretions can be sampled from these patients via from a sputum trap.

 

Salivary contamination is still a problem with endotracheal tubes, as the cuff can leak. Samples can therefore be contaminated by saliva, as evidenced by epithelial cells and upper resp flora. Bronchoalveolar lavage can also be done.

Lab Processing

Sputum gram stains can be done quickly, yielding important information. However, for definitive identification, and antibiotic susceptibility testing, more time is usually required.

 

Samples are immediately gram stained and are graded for quality using a Q score, ranging from 0 (extremely poor) to 3 (very good) quality. This score assesses epithelial cells, ciliated respiratory cells, and neutrophils.

 

Samples of sufficient quality or clinical importance are plated on a number of media, including blood agar (for isolation of gram positive cocci), MacConkey (for isolation of aerobic gram negative rods), chocolate (for isolation of Hemophilus), and legionella if this pathogen is suspected.

 

Antibiotic susceptibility can also be carried out, as appropriate.